Goro Yoshizaki — wants to breed bluefin tuna from mackerel.

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Bluefin Tuna Lou Linetty – 16lb King Mackerel Photo of King Mackerel 24.5 lbs

Taken From An Article By Olivia Judson, The New York Times – To a surprising extent, lifeforms are a kind of fancy Lego, built from interchangeable parts. This is true at the genetic level: copy a gene from a jellyfish and put it into the genome of a rabbit, a mackerel, or an alfalfa plant, and the organism will incorporate the gene and make the jellyfish protein. But it is also true for cells. Let me give you an example.

Primordial germ cells are the cells that will one day become eggs or sperm. They form early in the development of the embryo, and migrate to the part of the embryo that will become the gonads. In males primordial germ cells develop into what are known as spermatogonia — the cells that generate sperm cells throughout the male’s life. It turns out that you can transplant either of these cell types — the primordial germ cells, or the spermatogonia — from one species to another. For instance, if you transplant spermatogonia from a rat into the testes of a mouse, the mouse will begin to make rat sperm. Yet rats and mice have been evolving independently for more than 10 million years. Even more remarkable, in a rat, sperm take longer to develop: rats take 52 days to make sperm from a given spermatogonial cell, mice take 35.

Under most circumstances — spermatogonia included — you can’t transplant cells from one animal to another, because the immune system of the recipient gets in the way. After all, the immune system’s job is to round up and destroy intruders — and alien cells definitely count as intruders. This is why organ transplants between two beings of the same species often don’t work — and why attempts to transplant organs from one species to another have been a complete failure. In general, then, for foreign cells to establish themselves, the recipient animal must not have a working immune system.

There are a couple of ways to get around this. The first is to transplant cells into adult animals that don’t have immune systems; and there are special breeds of laboratory mice that do not. These were used for the rat sperm experiment. The second is to transplant cells into embryos, putting them into the recipient’s body before the immune system has matured and switched on. Then, the alien cells are simply accepted as being part of the body. Which is what Yoshizaki and his team have been doing with fish.

These researchers have done a series of transplantation experiments on rainbow trout (Oncorhynchus mykiss) and masu salmon (Oncorhynchus masou) — a pair of species that began to diverge at least 8 million years ago. Their most recent and surprising result is that spermatogonia from rainbow trout will, if injected into the body cavity of a salmon embryo, migrate to the gonads of the developing salmon and colonize them. What’s more, if the salmon is female, the spermatogonia will turn into eggs.

This is fascinating. It shows that spermatogonia are not already programmed to make sperm; instead, they obey signals from the surrounding cells in the embryo. So even though the transplanted cells were taken from an adult male rainbow trout, they retain the ability to swing both ways: they become sperm in a male salmon and eggs in a female. And they do this despite the millions of years that rainbow trout and masu salmon have been evolving independently.

But although the transplanted trout cells obey the instructions they get from the salmon body they’ve moved into, and (usually) become incorporated into that body, the genes inside them are still trout genes. Which means that when the salmon spawned, they produced trout fry. These trout were normal trout, with normal fertility.

(Here’s a strange wrinkle. Like humans, male salmon and trout have an X chromosome and a Y chromosome, and females have two Xs. Under normal circumstances, each egg contains one X chromosome; each sperm contains either an X or a Y. But what happens when you put spermatogonia into a female? Some eggs get a Y chromosome. In humans, YY males — the result of fertilization of a Y-bearing egg by a Y-bearing sperm — are impossible because the human Y chromosome is puny and genetically depauperate compared to the X. In fish, however, the Y chromosome looks broadly similar to the X, and seems to contain many of the same genes. Thus, YY males are regular guys.)

Why, why would anyone want to do all this? Yoshizaki’s hope is that his work will help relieve fishing pressure on species, like bluefin tuna (Thunnus thynnus), that grow relatively slowly and cannot presently be bred in captivity. One solution — the obvious one — would be to stop eating tuna. But this seems unlikely to happen. And since mackerel (Scomber japonicus) are small — you can keep them in fish tanks — and fecund — one female can produce 500,000 eggs in one spawning — transplanting bluefin tuna spermatogonia into mackerel embryos could be a way to breed lots of bluefin tuna quickly.

The experiment has not yet been done, and it may not work. Bluefin tuna and mackerel are more evolutionarily distant than masu salmon and rainbow trout; perhaps the differences between them will prove too big. (Human spermatogonia survive in mice, but — so far — have not been induced to produce sperm. This is not surprising: the most recent common ancestor of mice and humans lived more than 75 million years ago, which is before the dinosaurs went extinct.)

But never mind the uses. What I like about experiments like this is that they show we are all made of the same stuff. Cells from one species can be moved into another, and become incorporated, despite several million years of independent evolution. It’s a profound testament to our common ancestry.